Methods for fabricating self-aligning semicondutor heterostructures using silicon nanowires

ABSTRACT

Methods for fabricating self-aligned heterostructures and semiconductor arrangements using silicon nanowires are described.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 61/262,843, filed on Nov. 19, 2009 which is incorporated herein byreference in its entirety. The present application may be related toU.S. patent application Ser. No. 12/712,097 for ‘Methods for FabricatingHigh Aspect Ratio Probes and Deforming High Aspect Ratio Nanopillars andMicropillars’ filed on Feb. 24, 2010, U.S. patent application Ser. No.12/824,128 for ‘Method for Fabricating Micro and Nanostructures in aMaterial’ filed on Jun. 25, 2010, U.S. patent application Ser. No.12/711,992 for ‘Methods for Fabrication of High Aspect RatioMicropillars and Nanopillars filed on Feb. 24, 2010, and U.S. patentapplication Ser. No. 12/822,109 for ‘Methods for Fabricating PassivatedSilicon Nanowires and Devices Thus Obtained’ filed on Jun. 23, 2010, allof which are incorporated herein by reference in their entirety.

STATEMENT OF GOVERNMENT GRANT

This invention was made with government support under Grant No.HR0011-04-1-0054 awarded by the DARPA, Grant No. FA9550-04-1-0434awarded by the AFOSR and Grant No. W911NF-07-1-0277 awarded by the ARO.The government has certain rights in the invention.

FIELD

The present disclosure relates to fabrication using silicon nanowires.Moreover, it relates to methods for fabricating self-aligningarrangements on semiconductors.

BACKGROUND

Defining high aspect ratio structures with controllable sidewalls insilicon has become increasingly important both in the nanometer andmicrometer scale for solar cells, microelectronic devices, and chemicalanalysis. High aspect ratio micrometer pillars are used for solar cellinvestigations while nanometer scale high aspect ratio pillars areenabling fundamental investigations in theories of nanoscale pillarstress mechanics, silicon based lasers, and nanoscale electronic devicessuch as finFETs. Currently various nanofabrication techniques exist thatrely on self assembly or bottom-up processing. Some top-down processingenabling reproducibility in nanofabrication can also be found.

Further applications are high surface area chemical sensors, mechanicaloscillators and piezo-resistive sensors. High aspect ratio pillars withdiameters between 50-100 nm could prove useful for core-shell typeplasmonic resonators while pillars with sub-10 nm diameters have shownpromising light emission characteristics.

SUMMARY

According to a first aspect, a method of fabricating self-aligningelectronic components, the method comprising providing a substrate withone or more nanowires on a first side of the substrate and a firstconductive layer on a second side of the substrate, coating thesubstrate and the one or more nanowires with an insulator, and cleavingthe coated one or more nanowires to expose a selected length of anon-cleaved nanowire portion and a selected length of a non-cleavedinsulator portion.

According to a second aspect, a method of fabricating self-aligningelectronic components, the method comprising providing a substrate withone or more nanowires on a first side of the substrate and a firstconductive layer on a second side of the substrate, coating thesubstrate and the one or more nanowires with an insulator, depositing asecond conductive layer on the insulator, depositing a dielectric layeron the second conductive layer, and cleaving the coated one or morenanowires to expose a selected length of a non-cleaved nanowire portionand a selected length of a non-cleaved insulator portion.

According to a third aspect, a electronic arrangement comprising asemiconductor substrate, a cleaved semiconductor nanowire on thesemiconductor substrate, and an oxide layer partially coating thecleaved semiconductor nanowire and coating the semiconductor substrate,wherein the oxide layer and the cleaved semiconductor nanowire define anuncoated region of the cleaved semiconductor nanowire, an epitaxialmaterial on the uncoated region of the cleaved semiconductor nanowire.

According to a fourth aspect, an electronic arrangement comprising asemiconductor substrate, a cleaved semiconductor nanowire on thesemiconductor substrate, an oxide layer partially coating the cleavedsemiconductor nanowire and coating the semiconductor substrate, whereinthe oxide layer and the cleaved semiconductor nanowire define anuncoated region of the cleaved semiconductor nanowires, a firstconductive layer on the oxide layer, a dielectric material on the firstconductive layer, and a second conductive layer on the cleavedsemiconductor nanowire and the oxide layer.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent disclosure and, together with the description of exampleembodiments, serve to explain the principles and implementations of thedisclosure.

FIG. 1 shows an array of nanostructures.

FIGS. 2A-2E shows a cross-sectional view of a nanostructure with an endportion of an oxide layer etched back.

FIGS. 3A-3E show fabrication steps of growing a self-alignedheterostructure in accordance with an embodiment of the presentdisclosure. In particular:

FIG. 3A shows an exemplary cross-sectional view of a nanowire substrate.

FIG. 3B shows an exemplary cross-sectional view of an oxide passivatednanowire.

FIG. 3C shows an exemplary cross-sectional view of a nanostructure beingmechanically cleaved.

FIG. 3D shows a cross-sectional view with epitaxial material depositedon the mechanically cleaved surface.

FIG. 3E shows a cross-sectional view with a conductive layer depositedon a topside and a backside on the substrate.

FIG. 4 shows an exemplary perspective view of a cleaved nanostructure ona substrate.

FIGS. 5A-5E show methods of fabricating a self-aligned transistor inaccordance with an embodiment of the present disclosure. In particular:

FIG. 5A shows an exemplary cross-sectional view of an oxide passivatednanowire.

FIG. 5B shows an exemplary cross-sectional view of a substrate and theoxide passivated nanowire deposited with a conductive layer and adielectric layer.

FIG. 5C shows an exemplary cross-sectional view of a nanostructure beingmechanically cleaved.

FIG. 5D shows an exemplary cross-sectional view of deposition of aconductive layer on the cleaved nanostructure and substrate, and on thebackside of the substrate.

FIG. 5E shows an exemplary cross-sectional view with the topsideconductive layer etched back and a backside conductive layer.

FIG. 6 shows an exemplary perspective view of a cleaved nanostructure ona substrate where the substrate is coated with a conductive and adielectric layer.

DETAILED DESCRIPTION

Methods for fabrication of nanoscale electronic components are describedin accordance with various embodiments of the present disclosure.Nanowires can be fabricated, by way of example and not of limitation, byperforming standard photolithographic or electron-beam lithographictechniques, self-assembly to prepare masks for arrays, use oflithography to pattern catalysts and bottom-up techniques such asvapor-liquid-solid (VLS) growth instead of etching. For the sake ofsimplicity, throughout the present disclosure, the term ‘nanopillar’intends to indicate a substantially upright nanoscale shaft where theheight is much greater than the width (e.g., 5-10 times greater than thewidth) and can be used interchangeably with the term ‘nanowire’. Theterm ‘nanostructure’ intends to indicate the nanopillar including anyinsulator or conductive layer(s) deposited thereon. ‘Nanoscale’ isdefined herein to be any structure between 1 nm and 500 nm in width.

Photolithography is a process used in microscale fabrication toselectively remove parts of a film or bulk of a substrate. It uses lightto transfer a geometric pattern from a photo mask to a light-sensitivechemical called a photo resist on the substrate. Similarly, electronbeam lithography is a process where a beam of electrons are scanned in apatterned fashion to the electron-beam resist. This is followed by aseries of chemical treatments in a process similar to dark roomprocessing for photography. The photo or electron-beam resists can beutilized as a mask directly, or utilized to pattern a harder mask whichcan have better resilience as compared to masking directly.Electron-beam resist can be utilized to fabricate a patterned aluminumoxide (alumina) mask, then removing the electron-beam resist andutilizing the patterned alumina during etching. Lithography and highlyanisotropic etching enables routine fabrication of 30-50 nmnanostructures (100) in silicon with over 40:1 aspect ratios as shown inFIG. 1. Such structures can be further reduced in diameter by asubsequent thermal oxidation, wherein the oxidation process can bedesigned to self-terminate such that nanoscale pillars below 10 nm inwidth can be defined, allowing wide processing latitude.

Field effect transistors (FETs) are generally fabricated using abottom-up method, planar to the substrate. In a bottom up method, thebottom-most layer is deposited first, followed by precise alignment ofsubsequent layers, one on top of another until a desired pattern isultimately achieved. The final patterned arrangement becomes anelectronic component, such as transistors (e.g, FETs, MOSFETs, etc). Forexample, FETs can be fabricated by lithography and the geometry of suchFETs allow for charges to be conducted through a channel located below agate, from source to drain. In such arrangement, the channel is locatedin-plane to the fabricated surface of the substrate, where the substratecan be a silicon wafer. When a bias is applied to the gate, current iselectrostatically controlled between the source and the drain.

In recent applications, as shown for example in U.S. patent applicationSer. No. 12/822,109 filed on Jun. 23, 2010, incorporated herein byreference in its entirety, vertical methods for fabricating such FETsare described. Controlling the oxidation process can produce strainedsilicon nanowires used in the fabrication of the FETs. In such verticalgeometry, a nanowire is grown, oxidized, and the circumference is coatedby a gate electrode.

FIGS. 3A-3E show various steps of fabricating a semiconductorarrangement involving a self-aligning heterostructure in accordance withthe present disclosure. The term ‘self-aligned’ as used herein intendsto indicate alignment that occurs automatically, without relying on anoperator's skill or accuracy of a machine to accomplish the alignment. Aperson skilled in the art will understand that the number of stepsinvolved is only indicative and that the process can occur in more orfewer steps according to the various embodiments.

FIG. 3A is a cross-sectional view of a patterned, or etched substrate(310) comprising a substantially vertical nanopillar (320). By way ofexample and not of limitation, the substrate (310) and the nanopillar(320) are made of silicon (Si). An example method of fabricating suchnanopillars is described in U.S. patent application Ser. No. 12/824,128filed on Jun. 25, 2010 and U.S. patent application Ser. No. 12/711,992filed on Feb. 24, 2010, both incorporated herein by reference in theirentirety. As an alternative to this embodiment, the vertical nanopillarcan be fabricated on silicon-on-insulator (SOI) instead of bulk siliconstructure.

FIG. 3B is a further cross-sectional view of the substrate (310) and thenanopillar (320) covered by an insulator, which can be an oxide layer(330), e.g., silicon dioxide (SiO2) or other dielectric. The oxidationprocess can expand the silicon lattice by approximately 40% toincorporate oxygen. Such expansion leaves the adjacent un-oxidizedportion of the silicon under tensile strain. In nanowires, such straincan cause the oxidation process to automatically stop, ultimatelyresulting in a stable nanoscale tensile-strained silicon core with asilicon dioxide shell. Exposing the embedded silicon to very high strain(e.g., 2.5-3.0%) enhances the ability of such device to efficiently emitlight which can be utilized in, for example, opto-electronic switching.

U.S. patent application Ser. No. 12/822,109 filed on Jun. 23, 2010, alsoincorporated herein by reference in its entirety, describes methods toremove a portion of the oxide from the nanostructure to expose thesilicon nanowire located below, by first protecting portions of thenanostructure and the oxide layer on the planar portion of the substratewith some dielectric material, e.g., photoresist, and then strippingaway the oxide on the top end portion of the nanowire using methods suchas etching.

An alternative method of exposing the nanostructure is shown as across-sectional view in FIG. 3C, by mechanically cleaving (340) ormechanically polishing a portion of the nanostructure (360) to reveal apreviously unexposed non-removed portion (350) of the nanostructure.Mechanical cleaving results in a precise, clean cut of the nanostructurewith minimal difficulty relative to the etching methods previouslydescribed.

Once the nanostructure is cleaved, a single crystal current aperture(370) (400) surrounded by silicon dioxide (372) (410) is exposed, shownas a cross-sectional view in FIG. 3D and in a perspective view from thetop in FIG. 4. Since the aperture is based on the diameter of thesilicon nanowire, the aperture size is controlled by the lithographicpattern during the fabrication of the nanowire, and the temperature usedduring the oxidation process. By fabricating high aspect rationanostructures as described by way of example and not of limitation inU.S. patent application Ser. No. 12/711,992 filed on Feb. 24, 2010, U.S.patent application Ser. No. 12/712,097 filed on Feb. 24, 2010, and U.S.patent application Ser. No. 12/824,128 filed on Jun. 25, 2010, circularnano-apertures and sub-nano-apertures with diameters in the range ofapproximately 5 nm can be made. It is to be noted however, that thediameters are not limited to just the 5 nm range and can be larger orsmaller.

Epitaxial material (374) (e.g., GaAs, InP, InAs, Ge, or other III-Vmaterial) can be grown precisely within the current apertures and on topof the clearly defined, cleaved silicon nanowire aperture surface asshown in FIG. 4 by exploiting the exposed clean silicon aperture (400)as a template for re-growing. Re-growing such epitaxial material on theaperture (400) involves layering of dissimilar lattice structurecrystals in nanoscale environment, which ultimately creates quantumdots. For example, the crystal lattice of the nanowire substrate can bea silicon crystal and the re-grown quantum dot can be a GaAs crystal,one on top of another. However, such lattice structures can be grownwithout fear of defects in the lattice structure since the strainbetween the silicon and the re-growth material do not build up in thenarrow nanoscale aperture. Thus, strain is kept to a minimum and thedimensions of the quantum dots are less than the pseudomorphic distancein any direction.

Accurate re-growing of quantum dots allow for precisely controlled lightemission and electronic attributes. Additionally, simple definition oftwo and three terminal electrical and optoelectronic components can beaccomplished on silicon substrates as shown in FIG. 3E.

FIG. 3E shows a cross-sectional view of the cleaved nanostructure with aquantum dot (392), which is then coated with a conductive layer (390)(e.g., gold, silver, copper, etc.). The backside of the substrate canalso be coated with a conductive layer (380). Both conductive layersbecome contacts for the growth aperture arrangement.

The steps described from FIGS. 3A-3E describe methods of fabricatingheterostructures on a substrate where the heterostructures areself-aligned. The term ‘self-aligned’ as used in the present disclosureintends to indicate alignment that occurs automatically, without relyingon an operator's skill or preciseness of a machine to accomplish thealignment.

The present disclosure also describes methods for fabricating gate on asilicon nanostructure. However, the nanostructure is fabricated withtight control over gate length by initially fabricating thenanostructure to a length substantially taller than desired, thendepositing a precisely controlled protective spacer layer, andsubsequently cleaving or polishing the protruding portions of thenanostructure to obtain tightly controlled gate lengths. As a result,the need for precise manual alignment is eliminated as may be requiredin traditional lithography, thus resulting in gates which areself-aligned. Such fabrication is not limited to the fabrication of FETsbut can also be applied to fabrication of other nano-arrangements, byway of example and not of limitation to, light emitting diodes (LEDs),detectors and junctions, on silicon.

FIGS. 5A-5E describe methods of fabricating a FET with self-aligninggates. Self-aligned FETs eliminate the need to rely on the skill of aperson or a machine to precisely obtain proper alignment. FIG. 5A showsa cross-sectional view of an oxidized semiconductor nanowire (502),similar to FIG. 3B.

FIG. 5B shows a cross-sectional view of a nanostructure where a layer ofconductive material (510) is deposited on the surfaces of thenanostructure and the substrate. The conductive layer material can be,by way of example and not of limitation, gold, silver, copper, aluminumor other types of metals. The conductive layer can be deposited bysputtering, whereby the operator can precisely control the thickness ofthe conductive layer being deposited. Controlling the thickness of theconductive layer can be accomplished with a higher degree of precisionas compared to defining lithographic features at nanoscale levels.Therefore, arrangements with greater precision can be fabricated.

Contacts (220) (390) (570) in FIGS. 2E, 3E and 5E, respectively, areoften made on the nanowires in order to make them into functionalcomponents. One such method for making contacts can be accomplished byair-bridging, whereby polymer (500) is used to planarize the nanowiresbefore stripping off the top oxide (510) with hydrofluoric acid orselective dry etch.

In a further embodiment, the nanostructure is mechanically cleaved (530)as shown with a cross-sectional view in FIG. 5C. A dielectric layer(520) is deposited on the conductive layer (510) to protect theconductive layer (510) and to electrically isolate the conductive layer(510) from another conductive layer that will be deposited and describedin the following paragraphs. Mechanically cleaving (530) thenanostructure just above the protective dielectric layer (520) removes amajority of the top portion of the nanostructure, leaving behind a smallnanostructure (560) below the dielectric layer (520), thus exposing theremaining nanowire (600) with the insulator (610) completely surroundingthe perimeter of the nanowire as show in FIG. 6 with a perspectivetop-view.

Cleaving the nanostructure cleaves the gate length of the arrangement,thus the gate length can be precisely fabricated with ease. The gate isessentially ‘self-aligned’ since the alignment process typically used inlithography is no longer required.

Finally in FIG. 5D, a second conductive layer (540) is deposited on thedielectric (520) and the remaining portion of the nanostructure (560). Abackside contact layer (550) is also applied on the opposite side of thesemiconductor substrate. The top conductive layer (540) is etched, suchthat the top conductive layer remains as contact points (570) above thenanostructure (560), as shown in FIG. 5E.

Such arrangement creates a metal-oxide-semiconductor (MOS) structure,which can be, but is not limited to a MOSFET. In case of a FET or aMOSFET, the top contact and the backside contacts become the source anddrain, or the drain and source.

Although specific elements such as silicon, oxide, silicon oxide, etcwere used to describe the various features and embodiments of thepresent disclosure, a person or ordinary skill in the art wouldunderstand that other elements can be used in place.

The examples set forth above are provided to give those of ordinaryskill in the art a complete disclosure and description of how to makeand use the embodiments of the methods for fabricating self-aligningarrangements on semiconductors of the disclosure, and are not intendedto limit the scope of what the inventors regard as their disclosure.Modifications of the above-described modes for carrying out thedisclosure may be used by persons of skill in the art, and are intendedto be within the scope of the following claims. All patents andpublications mentioned in the specification may be indicative of thelevels of skill of those skilled in the art to which the disclosurepertains. All references cited in this disclosure are incorporated byreference to the same extent as if each reference had been incorporatedby reference in its entirety individually.

It is to be understood that the disclosure is not limited to particularmethods or systems, which can, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used in this specification and the appended claims, thesingular forms “a,” “an,” and “the” include plural referents unless thecontent clearly dictates otherwise. The term “plurality” includes two ormore referents unless the content clearly dictates otherwise. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which the disclosure pertains.

A number of embodiments of the disclosure have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the presentdisclosure. Accordingly, other embodiments are within the scope of thefollowing claims.

We claim:
 1. A method of fabricating self-aligning electroniccomponents, the method comprising: providing a substrate with one ormore nanowires on and in contact with a first side of the substrate anda first conductive layer on and in contact with a second side of thesubstrate, wherein the one or more nanowires comprises at least one of asilicon-on-insulator or a bulk silicon structure; substantially coveringthe substrate and the one or more nanowires with an insulator havingthickness that is less than a height of the nanowire, the insulatorcontacting the one or more nanowires and the first side of thesubstrate; cleaving the coated one or more nanowires to expose aselected length of a non-cleaved nanowire portion and a selected lengthof a non-cleaved insulator portion; and epitaxially depositing a quantumdot directly on the selected length of the non-cleaved nanowire portion.2. The method of claim 1, further comprising depositing a secondconductive layer on the substrate, the selected length of thenon-cleaved nanowire portion, and the quantum dot.
 3. The method ofclaim 2, wherein the substrate is a silicon substrate.
 4. The method ofclaim 3, wherein the insulator is a silicon dioxide insulator.
 5. Themethod of claim 4, wherein the cleaving is performed by mechanicalpolishing.